1. Field of the Invention
This invention relates generally to radiation therapy equipment for the treatment of tumors or the like with a radiation beam, and in particular to a detector array used with such equipment for precisely characterizing the intensity of the radiation beam over an irradiated area.
2. Background Art
Medical equipment for radiation therapy treats tumorous tissue with high energy radiation. The dose and the placement of the dose must be accurately controlled to insure both that the tumor receives sufficient radiation to be destroyed, and that damage to the surrounding and adjacent non-tumorous tissue is minimized.
External-source radiation therapy uses a radiation source that is external to the patient, typically either a radioisotope, such as .sup.60 Co, or a high powered x-ray source, such as a linear accelerator. The external source produces a collimated beam directed into the patient toward the tumor site.
Determining the radiation profile of the external radiation beam (i.e., the variation in intensity over the area of the beam) is important in planning the radiation therapy, or verifying assumptions concerning beam homogeneity. The ability to accurately characterize the beam profile also permits evaluation of any filters or attenuating blocks that may be used in conjunction with the radiation beam to control and attenuate the radiation beam.
Several methods are currently used to determine the intensity of a radiation beam over an exposed area. In one method, a tank of water simulates patient absorption and a small radiation detector, typically an ionization chamber, is driven to different positions within the tank while measurements are made. The ionization chamber generates a current between two charged electrodes, an anode and cathode, as conducted by atoms of a gas held in a cell between the electrodes and ionized by the radiation.
In order to have sufficient resolution in the measurement of the radiation beam, a number of locations in the beam must be sampled and the detector must be small. But the movement of the detector to each measurement point is time consuming and the small detector requires a significant period of time at each measurement point to collect enough energy to ensure precise measurement of the radiation intensity. The extended time required to collect these measurements may be inconvenient and may tax the operating limits of the radiation source (particularly, a linear accelerator) which must be active for a longer period of time than is usual for treatment. The setting up and taking down of the water tank used with this procedure, is also inconvenient.
A second method of determining the intensity of a radiation beam over an area uses multiple ionization chambers to characterize the beam at numerous measurements points simultaneously. Each ionization chamber is equipped with its own integration and amplification circuitry to provide a signal indicating the total radiation received. The circuit components must be closely matched to ensure comparable readings from each of the multiple ionization chambers. For large numbers of ionization chambers, this matching becomes difficult and the detector becomes quite expensive.